Electrochemical cell stack

ABSTRACT

A cell stack for use in a battery or fuel cell using a liquid electrolyte. A plurality of individual frames made from electrically insulating material, each having a cavity extending transversely therethrough and separate elongated supply and return electrolyte passageways leading to and from the cavity. An electrode assembly is disposed in a recess provided in one surface of one of the frames and sandwiched between it and another frame. Each frame is bonded to the next adjacent frame to provide a complete seal surrounding the periphery of the cavity, each group of two adjacent electrode assemblies and three frames providing an electrochemical cell. The frames may be injectionmolded from polysulfone and solvent-bonded by a chlorinated hydrocarbon.

United States Patent 1 Unkle, Jr. et a].

1451 Feb. 20, 1973 [54] ELECTROCHEMICAL CELL STACK [75] Inventors:Truman F. Unkle, Jr., Poway; John F. Loos, San Diego, both of Calif.

[73] Assignee: Gulf Oil Corporation [22] Filed: June 25, 1970 21 Appl.No.: 49,696

Primary Examiner-Allen B. Curtis [5 7] ABSTRACT A cell stack for use ina battery or fuel cell using a liquid electrolyte. A plurality ofindividual frames made from electrically insulating material, eachhaving a cavity extending transversely therethrough and separateelongated 'supply and return electrolyte passageways leading to and fromthe cavity. An electrode assembly is disposed in a recess provided inone surface of one of the frames and sandwiched between it and anotherframe. Each frame is bonded to the next adjacent frame to provide acomplete seal surrounding the periphery of the cavity, each group of twoadjacent electrode assemblies and three frames providing anelectrochemical cell. The frames may be injection-molded frompolysulfone and solventbonded by a chlorinated hydrocarbon.

3 Claims, 6 Drawing Figures PATENTEB FEBZO 1973 SHEET 2 OF 2 HII WM z/ rg5 ELECTROCHEMICAL CELL STACK This invention relates to electrochemicalcell stacks and more specifically to stacks of electrochemical cellsusing a liquid electrolyte and to methods for making such cell stacks.

Electrochemical devices, such as batteries and fuel cells, have longemployed a plurality of individual electrochemical cells which areelectrically interconnected in a manner to produce the voltage-amperagecharacteristics desired in such an electrical power providing device.Various methods have been devised to incorporate a plurality ofelectrochemical cells into a composite cell stack; however, many ofthese methods have not been considered suitable for a production lineoperation. When a liquid electrolyte is employed with the cell stack andparticularly when that electrolyte is circulated throughout the cellstack, there are additional problems attendant to the design of a cellstack because of the potential for leaks at locations where theelectrolyte enters and leaves each of the electrochemical cells.Accordingly, improved methods for making cell stacks of this generaltype and improved designs are desired which take into consideration theabovementioned factors.

It is an object of the present invention to provide an improved cellstack utilizing a plurality of electrochemical cells and a circulatingliquid electrolyte. Another object of the invention is to provide animproved method for making an electrochemical cell stack designed toemploy a circulating liquid electrolyte. A further object is to providean improved cell stack utilizing electrode subassemblies which are heldin frames of electrically insulating thermoplastic material.

These and other objects of the present invention will be apparent fromthe following detailed description of a cell stack and methods formaking same when read in conjunction with the accompanying drawingswherein:

FIG. 1 is a perspective view of an electrochemical cell embodyingvarious features of the invention;

FIG. 2 is an exploded perspective view, reduced in size, of the cellstack shown in FIG. 1;

FIG. 3 is a vertical sectional view taken generally along the line 33 ofFIG. 1;

FIG. 3A is a fragmentary enlarged view of the upper portion of FIG. 3;

FIG. 4 is a horizontal sectional view taken generally along the line 4-4of FIG. 1; and

FIG. 5 is an enlarged elevation of one of the individual frames shown inFIG. 2.

Very generally, a method is provided for making cell stacks ll of anynumber of electrochemical cells by serially uniting a plurality ofindividual assemblies each of which assemblies contain an electrodesubassembly 13 that is supported in a surrounding frame 15 ofthermoplastic material. The thermoplastic frames 15 are designed topermit inexpensive manufacture, as by injection molding, and each hasformed in its front surface an open trough system for carrying liquidelectrolyte to and away from the individual electrochemical cells, thebounds of which are generally defined by the frames. Each trough systemconnects with a pair of openings 17,19 in the individual frames whichwhen assembled constitute an electrolyte inlet 21 and outlet 23 for allof the cells in the cell stack.

The rear surface of each individual frame 15 is flat in the regionswherein, in the front surface thereof, this trough system is located.Accordingly, when the frames 15 are assembled with one another, the opentrough system is transformed to a closed supply passageway 25 and aclosed return passageway 27. The frames 15 are suitably bonded,preferably by using an organic solvent for the thermoplastic material,to seal one to the next in the regions bordering the trough system andaround the entire periphery of the frames. When two frames 15 are bondedto each other, the electrode subassembly 13 is sealed in its operatingposition, sandwiched between two frames. Addition of another frame 15and electrode subassembly 13 to the three-component sandwich completesone electrochemical cell, which is made up of single electrodes from twoadjacent electrode subassemblies 13. A cell stack 11 of any desirednumber of electrochemical cells can be constructed in this relativelysimple manner wherein the individual assemblies are handled and addedone at a time.

In the illustrated cell stack 1 l, as perhaps best seen in FIGS.- 1 and2, there are depicted a front end plate 29 and a rear end plate 31 whichhave three frames 15 disposed therebetween. This illustrated cell stack11 includes three electrochemical cells arranged in series, with oneelectrochemical cell being located generally in the region defined byeach of the frames 15, as will be more clear hereinafter. It should beunderstood that such a three-cell arrangement is shown primarily forpurposes of illustration and that, in actuality, cell stacks using alarger number of cells, for example 15 cells, connected in series wouldlikely be employed in order to provide the higher electrical powergenerating capabilities desired for most commercial applications.However, the principles of the cell stack assembly and operation will bereadily apparent from the following description of illustratedthree-cell arrangement.

The electrolyte inlet 21 and the electrolyte outlet 23 are located nearthe bottom of the cell stack and extend parallel to each othertransversely therethrough. The electrolyte inlet 21 is composed of thealigned openings 17 which are provided in the end plates 29, 31 and ineach of the frames 15. Similar aligned openings 19 constitute the outlet23 in the assembled cell stack. The illustrated cell stack 11 isdesigned for operation with electropositive metal electrodes, of amaterial such as zinc, and with electrodes utilizing a gaseouselectronegative material, such as oxygen.

In the illustrated embodiment, oxygen is supplied to the electrodesubassemblies 13 either in the form of oxygen or air, and accordingly agas inlet 33 is provided in the upper portion of the front end plate 29.This inlet 33 leads to an internal passageway system 35 in the front endplate which is connected by suitable fittings 37 to both of theelectrodes of the first electrode subassembly in line. As best seen inFIGS. 3A and 4, these fittings 37 provide a gas to the front surface ofthe first electrode assembly, and each electrode assembly 13 includesintegral couplings 39 which extend from the rear face thereof andconnect to each subsequent electrode in line, thereby supplying oxygento each of the electrode subassemblies. Suitable O-rings 41 are providedadjacent each electrode subassembly and surrounding the fittings 37 andcouplings 39 to seal the oxygen passageways against leakage. Although,because of the location of the section line 44 (FIG. 1), only thelefthand gas distribution system is shown in FIG. 4, a similar gasdistribution arrangement is also provided for the righthand electrodesof the electrode assemblies 13.

Each of the electrode subassemblies 13 includes a pair of rectangularporous plates 43 which constitute the electrodes through which theoxygen is supplied to the electrolyte interface in the electrochemicalcells. These porous plates 43 are mounted in side-by-side arrangement byattachment in pairs to thin backing plates 45. The backing plates 45 aredeformed to provide a region, of somewhat smaller dimensions than theporous plates 43, between the plates which region serves as a plenumchamber 47. Each backing plate 45 is suitably electrically andmechanically connected to a porous plate 43 along a line surrounding theperiphery of this deformed region to seal the edges of the plenumchamber 47 so that the oxygen can diffuse only through the porous plate45. The porous plates 43 are made of a suitable electroconductivematerial, such as sintered nickel, and the thin metallic backing plates45 are made of a suitable metal, such as mild steel or nickel. Theplates may be electrically and mechanically joined by welding orbrazing.

The rectangular backing plates 45 are planar along their periphery and,as best seen in FIGS. 3A and 4, are seated in recesses 49 provided inthe rear surfaces of the frames 15. When an electrode subassembly 13 isseated in a frame 15, the pair of porous electrodes 43 protrude into andare located generally centrally within a pair of openings or vacantportions 51 of the frames which extend transversely therethrough. Tofurther assist in the desired spacing and positioning of the electrodesubassemblies, three generally vertically extending slots 53 are cut inthe central region of the backing plates 45. These slots 53 accept lugs55 provided on the frames and facilitate the assembly of the overallcell stack 1 1 as hereinafter indicated.

As earlier indicated, the intermediate frames 15 are designed to beinexpensively produced, as by injection molding, from thermoplasticmaterial, and they are formed with the two side'by-side vacant portions51 which, after assembly, each define an electrochemical cell region.The recesses 49 formed in the rear surface of the frames 15 accept andseat the peripheral edges of the backing plate 45 of an electrodesubassembly 13. The upstanding lugs 55 which fit into the elongatedslots 53 provided centrally in the electrode backing plates are formedon the rear surface of a vertical bar portion 57 of the frame 15 whichseparates the two vacant portions 51. Formed in the front surface of theframes 15 are the pair of open troughs 25,27 which respectively carrythe liquid electrolyte to and from the vacant portions 51 which definethe individual electrochemical cells.

More specifically, the supply passageway 25 is defined by a trough thatis in communication with the entrance opening 17 which constitutes partof the electrolyte inlet 21. As best seen in FIG. 5, the supplypassageway trough 25 first loops around the opening 17 and then extendsvia a straight section to the lefthand side of a distribution chamberportion 59 which underlies the two vacant portions 51 of the frame. Theelectrolyte supply passageway 25 is purposely designed with the longlength indicated to achieve the desired intra-cell electrical isolation.Previously, long individual inlet tubes were considered necessary tominimize self-discharge between cells, and it is considered to be anadvantage of the illustrated frame design that such long supply andreturn passageways are incorporated within the frame itself. To achievethe desired intra-cell isolation, it has been determined that theindividual passageways should be at least about 30 cm. long and have across section not greater than about 0.5 sq. cm. or the equivalent.

Assembly of the cell stack is also facilitated because the molded framesindividually contain the electrolyte supply and return passageways forall of the electrochemical cells, and now only two plumbing connectionsare required for the entire cell stack. Short vertical, tapered inletholes 61 are formed in the lower portion of each of the frames 15 andlead from the distribution chamber 59 upward into the vacant portion 51along the lower edge thereof. A pair of flow-directing baffles 63 aremolded in the frame 15 and the upper lefthand corner of the distributionchamber 59 to facilitate smooth flow therethrough.

The electrolyte flows upward through the electrochemical cells definedby the vacant portions 51 and exits therefrom via short holes 65 alongthe top edge of the vacant portions 51 which lead into an upper chamber67 formed in the front surface of the intermediate frames 15 which is aportion of the electrolyte return passageway 27. From the chamber 67,the return passageway 27 extends along a long vertical section leadingdownward and into the side of the exit opening 19. The long flow pathprovided by the trough which forms the electrolyte return passagewaylikewise preserves the desired intra-cell electrical isolation betweenthe series-connected cells in the cell stack 11.

Three tie-rod holes 69 are provided in the lower portion of theintermediate frames 15, and similar tie-rod holes 69 are also providedin the front and rear end plates 29,31. Three additional tie-rod holes69a are also provided in the end plates 29,31 at locations below thebottoms of the individual frames 15. One hole 69b is provided at the topof the front plate 29. Annular recesses 71 are provided in the frontsurface of end plate 29 (FIGS. 3 and 4) to receive O-rings. If it isdesired to couple one cell stack 11 to another, six tierods are insertedthrough the holes 69,69a and 69b, and O-rings are placed in the recessesto seal the junctions in the electrolyte passageways 21 and 23.

The electrode subassemblies 13 are dual-functional electrodes. Theporous plate 43 serves as the oxygen electrode, and zinc is depositedupon the exterior surface of the backing plates 45 to constitute theother electrode, as shown in dotted outline in FIG. 3A. As previouslyindicated, a cell stack 11 might include some 15 electrode assemblies 13and 15 mating frames 15 and thereby provide a 15-cell series-connectedsystem. The terminal assemblies 71,73 are respectively provided in thefront and rear end plates 29, 31 and brazed to end plate 75 and backplate 45 respectively, for making external electrical connections to theelectrochemical cells. Four separate terminal strips are connected by abus to provide a single external connection at the terminal 73. Thefirst electrode assembly 13 provides the porous nickel electrodes forthe front electrochemical cell and a separate support plate 75 isprovided, between the rear surface of the front end plate 29 and thefirst frame 15, upon which plate zinc is electrochemically deposited toconstitute zinc electrode for the first cell in line. As best seen inFIG. 4, the terminal assembly 71 includes a conductor, which may be athin nickel and silver-plated copper strip, located in the front endplate 29 which strip connects the support plate 75 to the externalterminal.

The bi-functional electrode subassemblies 13 electrically interconnectthe porous oxygen electrode of one cell and the zinc electrode of thenext adjacent cell in series throughout the cell stack 11. Recesses 79are provided in the forward face of the rear plate 31 wherein thefour-strip conductor network 81 resides which is part of the rearterminal assembly 73 and makes electrical contact with the rear surfaceof the backing plate 45 of the last electrode subassembly 13 in line.This backing plate 45 does not support a zinc deposit as do the backingplates of the other subassemblies. The four strips of the conductornetwork 81 extend exterior of the cell stack 11, as best seen in FIG. 4,where they are connected to a bus which serves as the terminal 73.Suitable gaskets 83 (FIG. 3A) may also be provided in the front face ofthe rear end plate 31 to assure a good seal around the periphery of therearmost electrode subassembly 13.

As one example of how assembly of the cell stack might be carried out,the front end plate 29 is first located, with its forward surface down,in an assembly jig having a shape that is proportioned to register withthe exterior contour of the frames 15. A pattern of bonding agent isthen applied to either the rear surface of the front end plate or thefront surface of the first intermediate frame 15. An organic solvent forthe thermoplastic material is preferably used as the bonding agent.However, other adhesives may be used as the bonding agent. Whenpolysulfone is used for the frames, a chlorinated hydrocarbon, such asmethylene chloride is preferably used as the bonding agent.

The solvent is conveniently applied by placing the intermediate frame15, with its front surface (as viewed in FIG. 5) face down on asaturated felt applicator. The single electrode support plate 75 is thendisposed atop the front end plate 29 and O-rings 41 are slipped over theends of the fittings 37 which protrude through the support plate 75. Theframe is disposed in surfaceto-surface contact with the rear surface ofthe front end plate 29. Pressure is applied and is maintained for aperiod of time sufficient for the solvent to evaporate, thus assuring agood bond between the first intermediate plate 15 and the front endframe 29.

An electrode subassembly 13 is now located in the upper surface of theframe presently in the assembly jig, and because the rear face of theintermediate frame 15 is disposed upward, alignment is facilitated ofthe edges and slots 53 of the electrode backing plate in the recesses 49and with the upstanding lugs of the intermediate frame. O-rings arefitted over the protruding portions of the couplings 39 which extendfrom the rear face of the electrode assembly. The front surface of thenext intermediate frame 15 is then treated with the solvent. It isdisposed in the jig, and pressure is again applied to unite it to theearlier-bonded frame and front end plate. Bonding of the second frame 15entraps the electrode subassembly 13 between the two frames. Repetitionof these steps is then carried out to sandwich another electrodesubassembly 13 between the second frame and a third frame. In commercialproduction, the cell stack 11 might be, for example, built up to employ15 such intermediate frames and electrode subassemblies.

After the last frame has been bonded in place, an electrode subassembly13', which is generally the same as those previously employed except forits gas passageway system, is positioned in the recesses 49 in the rearface of the third frame. As best seen in FIG. 4, the last electrodesubassembly 13' has no couplings 39 extending from the backing plate andincorporates a radially-slotted grommet 87, which maintains the desiredspacing between the porous electrode 43 and the backing plate 45 in theregion of the plenum chamber 47.

The forward surface of the rear end plate 31 then has the desiredsolvent pattern applied thereto, and with the current collector network81 and the two peripheral gaskets 83 in place, it is positioned atop thecomponents already in the assembly jig. The front and rear end plates29,31 of the cell stack are compressed together to complete the bondingoperation.

The cell stack 11 thus created is ready for operation after suitableelectrical connections are made to the terminals 71 and 73 extendingfrom the righthand side of the front and rear end plates 29,31. A sourceof oxygen under suitable pressure, for example 5-10 psig., is connectedto the gas inlet 33. The liquid electrolyte may, for example, be a 20percent by weight aqueous potassium hydroxide solution. Suitableplumbing connections are made to the electrolyte inlet 21 and outlet 23,and pumping is begun to fill the passageways 25,27 and the vacantportions 51 of the cell stack 11 with electrolyte and continuecirculation and recirculation of the electrolyte therethrough. T'oelectrically charge the cell stack 11, liquid electrolyte which issaturated with zinc oxide is pumped therethrough while appropriateelectrical potential is applied to the electrodes. This causes metalliczinc to be electrochemically deposited upon the surfaces of the backingplates 45 which are in contact with the electrolyte in the individualcells.

During the charging process, oxygen is liberated at the porous nickelelectrodes 43, and this oxygen is carried with the circulatingelectrolyte stream out of the cell stack where it is subsequentlyseparated, as by using a suitable gas-liquid separator. A more detailedexplanation of the electrochemical operation of the cell stack is setforth in U. S. Pat. No. 3,391,027, issued July 2, 1968 to J. T. PorterII. After zinc metal deposits of the desired amount have been built-upon the backing plates 45 and the support plate 75 within the centralregions of each of the frames 15 that constitute the individualelectrochemical cells (as indicated in phantom outline in FIG. 3A), thecell stack 11 is ready for operation. Thereafter, whenever an electricalload is connected across the terminals 71 and 73, and anoxygencontaining gas is supplied to the gas inlet 33 at a sufficientpressure to cause it to permeate through the porous plates 43 to theelectrolyte interface, and a sufficient circulation of electrolyte ismaintained throughout the cell stack 11 to carry away the zinc oxidereaction products, electrical power generation will occur.

It is considered that the invention provides a simple and an extremelyeconomical cell stack design which is particularly suited for productionline assembly. There are considerable advantages in both production andin performance which result from the solvent-bonding of one frame toanother, Assembly of the cell stack in this manner eliminates the needfor the employment of pressure plate and spring arrangements which wereheretofore used to maintain the integrity of and to prevent leaks fromoccurring in a multiple cell stack of this type. Moreover, whereas itwas previously considered necessary to employ a plurality of relativelylong tubes to make individual inlet and outlet connections to eachelectrochemical cell in a series-connected cell stack, in order toachieve electrical isolation between the cells of the stack, theillustrated frame design integrally incorporates passageways of the.desired length as a part of the trough system. This design obviatesthis multitude of individual connections, which were previously a sourceof potential leaks, by molding them as an integral part of the frame,and it further facilitates production assembly by minimizing the numberof plumbing connections.

Although the invention has been illustrated with regard to certainpreferred embodiments, it should be understood that modifications andchanges such as would be obvious to one having the ordinary skill ofthis art can be made without deviating from the spirit of the invention.Various of the features of the invention are set forth in the claimswhich follow.

What is claimed is:

1. A cell stack for use in an energy conversion system using a liquidelectrolyte, which comprises a plurality of metallic electrodeassemblies, each of said electrode assemblies including a porous plateto which an elec trochemically reactive gas is supplied to provideonehalf of the electrochemical couple and an impervious plateelectrically and mechanically bonded to said porous plate, portions ofsaid impervious plate being spaced from said porous plate to provide agas plenum chamber therebetween, said impervious plate being adapted tosupport a deposit of an electropositive metal on the surface oppositefrom said plenum chamber, said electrochemical cells including a porousplate from one electrode assembly and an electropositive metal depositfrom the next adjacent electrode assembly, a plurality of frames madefrom an electrically insulating material, each of said frames having acavity therein which extends transversely therethrough and each alsoincluding separate elongated supply and return electrolyte passagewaysleading to and from said cavity, each electrode assembly being disposedin a recess provided in one surface of one of said frames, saidelectrode assemblies being sandwiched between adjacent pairs of saidframes, the surfaces of each said frames being bonded to said nextadjacent frame in a manner to provide a complete seal surrounding theperiphery of said cavity, said electrolyte supply and return passagewaysbeing in the form of open troughs in one surface of said frames whichare closed by the abutting surface of the adjacent bonded frame, saidsupply and return passageways being respectively in fluid communicationwith a pair of opening: extending transversely through said frame andemg bot disposed below said recess, said pairs of openings in all saidframes being located in aligned relationship in said cell stack andconstituting parallel electrolyte inlet and outlet conduits for all ofsaid electrochemical cells in said stack, said trough in each framewhich at its one end communicates with said transverse inlet Openingcompletely encircling said opening and communicating at its other endwith a distribution chamber leading to the bottom of said cavity, eachgroup of two adjacent electrode assemblies and three frames providing anelectrochemical cell.

2. A cell stack in accordance with claim 1 wherein said supply andreturn passageways are each at least about 30 centimeters in length andnot greater than about 0.5 cm. in cross section to assure intracellelectrical insulation.

3. A cell stack in accordance with claim 1 wherein said frames are madeof polysulfone and each frame is solvent-bonded to the next adjacentframe.

1. A cell stack for use in an energy conversion system using a liquidelectrolyte, which comprises a plurality of metallic electrodeassemblies, each of said electrode assemblies including a porous plateto which an electrochemically reactive gas is supplied to provideone-half of the electrochemical couple and an impervious plateelectrically and mechanically boNded to said porous plate, portions ofsaid impervious plate being spaced from said porous plate to provide agas plenum chamber therebetween, said impervious plate being adapted tosupport a deposit of an electropositive metal on the surface oppositefrom said plenum chamber, said electrochemical cells including a porousplate from one electrode assembly and an electropositive metal depositfrom the next adjacent electrode assembly, a plurality of frames madefrom an electrically insulating material, each of said frames having acavity therein which extends transversely therethrough and each alsoincluding separate elongated supply and return electrolyte passagewaysleading to and from said cavity, each electrode assembly being disposedin a recess provided in one surface of one of said frames, saidelectrode assemblies being sandwiched between adjacent pairs of saidframes, the surfaces of each said frames being bonded to said nextadjacent frame in a manner to provide a complete seal surrounding theperiphery of said cavity, said electrolyte supply and return passagewaysbeing in the form of open troughs in one surface of said frames whichare closed by the abutting surface of the adjacent bonded frame, saidsupply and return passageways being respectively in fluid communicationwith a pair of openings extending transversely through said frame andbeing both disposed below said recess, said pairs of openings in allsaid frames being located in aligned relationship in said cell stack andconstituting parallel electrolyte inlet and outlet conduits for all ofsaid electrochemical cells in said stack, said trough in each framewhich at its one end communicates with said transverse inlet openingcompletely encircling said opening and communicating at its other endwith a distribution chamber leading to the bottom of said cavity, eachgroup of two adjacent electrode assemblies and three frames providing anelectrochemical cell.
 2. A cell stack in accordance with claim 1 whereinsaid supply and return passageways are each at least about 30centimeters in length and not greater than about 0.5 cm.2 in crosssection to assure intracell electrical insulation.